1. Field of the Invention
The invention relates to a process for bonding and sealing substrates impermeable to water vapor with hotmelt adhesives or sealing compounds which post crosslink in the presence of moisture.
2. Statement of Related Art
Hotmelt adhesives are enjoying increasing popularity in the bonding field. They are solventless, and can be handled without pollution problems. In addition, they are suitable for production processes with short cycle times. The original disadvantage of hotmelt adhesives, namely the poor bond strength at elevated temperature, has recently been overcome by using as hotmelt adhesives reactive systems which when melted, react to provide materials which cannot be melted a second time or can be melted only at a much higher temperature.
One particularly important group of such a reactive hotmelt adhesives are the moisture-crosslinking hotmelts. Moisture-crosslinking hotmelts are generally understood to be solventless adhesives which, after application to a substrate, acquire their ultimate strength and thermal stability under load by subsequent hardening by the action of water.
It is also known that moisture-hardening hotmelts can be obtained by reacting free NCO groups of prepolymers with .alpha.-aminoalkyl trialkoxylsilanes, in which case the hardening reaction takes place through the terminal Si(OR).sub.3 groups, cf. H. F. Huber, H. Muller, 11th Munchener Klebstoff- und Veredelungsseminar, Octobar 1986. This lecture also disclosed a moisture-hardening hotmelt synthesized from a polyester containing free OH groups by initially reacting the OH groups with polyfunctional isocyanates in excess. The free NCO groups then present are subsequently reacted with .alpha.-aminoalkyl trialkoxysilanes to form materials known as silane hotmelts.
However, one disadvantage of hotmelt adhesives which require the presence of water, is that the hardening reaction cannot take place completely, if at all, if the substrates to be bonded are impermeable to water vapor. Accordingly, an attempt has been made (DE-A 34 12 884) to overcome this deficiency by using a primer consisting essentially of a solution of hexamethylene tetramine in acetone. However, the use of such a primer generally gives unsatisfactory results, particularly in the case of silane- or siloxane-crosslinking systems. In addition, in practice the amine emits an unpleasant odor.
Similar problems exist in the field of joint sealing compounds. An important group of joint sealing compounds are the mixtures containing polyurethane prepolymers which require the presence of and contact with atmospheric moisture to post-crosslink and harden. The polyurethane prepolymers used in polyurethane joint sealing compounds correspond, in their basic structure, to the polyurethane prepolymrs which are used for hotmelts.
In the application of polyurethane joint sealing compounds which post-crosslink in the presence of moisture, inadequate hardening or unsatisfactorily long hardening times are obtained when the joints to be sealed are bounded by substrates impermeable to water vapor.